1. Field of the Invention
This invention relates generally to an optical communication system and, more particularly, to a multi-rate variable duty cycle modem for use in an optical intersatellite link.
2. Discussion of the Related Art
Optical communication systems for use in spacecraft have heretofore been limited to much less than 0.5 watts of power. Although the availability of rare earth element doped fiber amplifiers has increased the potential to deliver more watts of power, there remains a focus on efficient use of power when designing these types of optical communication systems.
Fiber amplifiers also facilitate the implementation of external modulators in an optical communication system. External modulators generally allow the use of differential phase-shifted keyed (DPSK) modulation and other efficient modulation techniques at data rates from 1 to 13 Gb/s. Unfortunately, due to the lack of narrowband fiber-based optical filters, an optical communication system employing these fiber amplifiers becomes inefficient at data rates below 1 Gb/s. As a result, there is a need to design optical communications systems which are power efficient at rates lower than 1 Gb/s. In this way, optical communication systems used in the spacecraft environment are able to conserve power during periods of low data demand.
Therefore, it is desirable to provide an optical communication system for transmitting multi-rate data signals in a power efficient manner.
In accordance with the present invention, an optical communication system is provided for transmitting a multi-rate data signal between a transmitter and a receiver in a power efficient manner. The optical communication system includes an optical source that supplies an optical carrier signal; an encoder that receives and encodes a data signal; a modulator that modulates the optical carrier signal with the encoded data signal, such that the modulator varies the duty factor of the modulated optical signal based on the encoded data signal; and a fiber amplifier that amplifies the modulated optical signal prior to transmission by the transmitter. In accordance with the present invention, the modulation scheme of the optical communication system varies the duty cycle of the modulation to attain power efficiency during periods of low data demand.
In an alternative modulation scheme, data bits are encoded using bi-phase mark encoding and are transmitted in variable-length blocks. Although this alternative modulation scheme pays a power penalty, it enables the use of a common interferometric demodulator for all modulation rates. Moreover, the power penalty decreases with the number of bits transmitted in each block.